This invention relates to the use of polarized light to determine stress in transparent articles. More particularly, the present invention relates to an apparatus for measuring stress levels present in blown plastic bottles suitable for containing drinks or foods.
Analysis of stress distributions in models composed of transparent materials is commonly employed by engineers to verify calculations of stress concentration in large scale structures. Plane polarized light is directed to pass through transparent plastic models of a structure. As the plane polarized light passes through the model, the light breaks up at every point (birefringence) into two components corresponding to the directions of the two principal stresses at the point. If the light is then passed through another polarizing element (conventionally known as an analyzer), intensity variations or interference bands form that are indicative of principal stresses in the model. This method of stress determination is quite useful because it is non-destructive and can give both quantitative and qualitative information as to stress defects.
Stress determination in plastic bottles used as containers for food or drink is important to reduce failure rates of bottles. Many applications of plastic bottles require subjecting the bottles to high temperatures and pressures, both conditions capable of causing failure in a bottle weakened by a stress concentrating defect. Accordingly, determination of stress at an analysis point on a blown plastic bottle using polarized light and taking advantage of the birefringent properties of aligned polymers and polymeric crystals in blown plastic bottles would be advantageous. Plane polarized light passing through the bottle will travel at different velocities, with light polarized parallel to the optical axis of a polymeric crystal travelling at a different velocity than light polarized perpendicular to the optical axis of the crystal. When the light recombines after passing through the crystal, the light may be circularly, elliptically, or linearly polarized. Since light typically passes through several polymeric crystals as the light travels through the bottle wall, output is commonly elliptically polarized, being a linear function of polarization in the light path through the bottle wall.
The degree of elliptically polarized light passing through the bottle is a function of both the effective thickness and the birefringence of the polymeric crystals in the bottle wall. Changes in polarization are therefore dependent on wall thickness variations as well as stress variations. Since many crystals having differing orientations and birefringent properties are present in a light path, the initial linear polarization is almost destroyed, with a nearly random polarization of the emergent light. However, a small residual polarization typically remains that is indicative of stress and wall thickness variations. The intensity of polarization is measurable, and multiple measurements can be taken at a plurality of analysis points on the bottle wall to ensure consistency of the bottle wall and reduce chance of bottle wall failure.
Accordingly, the present invention provides a bottle stress analysis system using polarized light for determining stress levels of a transparent plastic bottle having a bottle wall that defines a bottle cavity. The bottle stress analysis system includes a rotation assembly for rotating the bottle and a polarized light source assembly insertible into the bottle cavity to direct plane polarized light through the bottle wall. An analyzer assembly is positioned outside the bottle for measuring intensity of light passing through the bottle wall, the analyzer assembly having a polarization analyzer positioned to transmit the light to a photodetector. In addition, a mechanism for rotating one of the polarized light source and the analyzer to determine maximum and minimum polarized light intensity at the photodetector, and consequently measure stress levels at the analysis point, is provided. It will be appreciated that the analysis point may be located at any area of the bottle including the base or finish and is not limited to merely the sidewall of the bottle.
In preferred embodiments the rotation assembly of the bottle stress analysis system includes a rotatable platter configured to support the bottle. A platter stepper motor is connected to the platter to rotate the platter a predetermined angular distance and briefly maintain a stationary position until maximum and minimum polarized light intensity is determined. A computer control assembly for automatically controlling operation of the platter stepper motor is also provided, with the computer control assembly controlling the angular distance through which the platter is rotated, and the duration the stationary position is maintained before further rotation. Motors other than a stepper motor could be used with corresponding control means to achieve substantially the same result.
To effectively provide a source of directed, linearly polarized light, the bottle stress analysis system includes polarized light source assembly. The polarized light source assembly includes a periscope insertible through a neck of the bottle into the bottle cavity, the periscope holding a light source and a polarizer that transmits plane polarized light, with the polarizer being positioned to transmit plane polarized light at a 45.degree. angle with respect to an axis of symmetry of the bottle. The light source can optionally be a light emitting diode (LED) connected to a light source intensity control to vary intensity of light emitted by the LED. A lens (or multiple lens system) for focusing light emitted by the light emitting diode, and a mirror for reflecting converging, plane polarized light toward a stress analysis point on the bottle coincident with the focal point are also positioned in the periscope. To further increase control of the system, the light source intensity control is connected to a computer control assembly for automatically controlling operation of the light source intensity control.
After light has passed from the periscope and through the bottle wall, the bottle stress analysis system utilizes an analyzer assembly for linearly polarizing the light. In addition to a polarizing element (the analyzer) the analyzer assembly includes a lens for focusing polarized light passing through the polarization analyzer onto the photodetector. The photodetector is configured to have a varying electric current in response to variations in light intensity, and further includes an electrical converter for converting the electrical current to an electrical voltage, and an amplifier for amplifying that voltage. A computer control assembly having an analog to digital converter converts this elecrical voltage output of the amplifier into computer readable digital format.
The rotating means for rotating the polarization analyzer includes an analyzer rotation assembly for rotating the polarization analyzer relative to a fixed plane of polarization of light emitted by the polarized light source assembly. An analyzer stepper motor is connected to the polarization analyzer to rotate the polarization analyzer a predetermined angular distance and briefly maintain a stationary position until maximum and minimum polarized light intensity is determined. Again, to increase ease of operation and speed of bottle stress analysis, a computer control assembly for automatically controlling operation of the analyzer stepper motor is provided. The computer control assembly controls the angular distance through which the polarization analyzer is rotated, as well as the duration the stationary position is maintained before further rotation.
The various features and advantages of the invention will become more apparent to those skilled in the art upon consideration of the following detailed description of a preferred embodiment exemplifying the best mode of carrying out the invention as presently perceived.